The majority of the known ion-exchange resins, particularly the alkylaminophosphonic resins with a styrene-divinylbenzene matrix, exhibit limited resistance to the successive swellings and contractions that occur during their passage from the regenerated form to the saturated form, and vice versa. The high percentage of cracked and broken beads which thus occurs in such resins during service severely limits their use, notably in very concentrated solutions which require a high mechanical resistance to osmotic shock. For this reason, their industrial application in such cases is unsatisfactory and sometimes even prohibitive.
French Patent Application No. 74.25610 as well as U.S. Pat. No. 4,002,564, the disclosure of which is hereby incorporated by reference, have as their subject alkylaminophosphonic chelating resins of the same type as the resin which is the subject of the present invention. However, the known resins do not exhibit a sufficiently high osmotic shock resistance at the same time as an adequate exchange capacity, so as to permit their economic use for the separation of metallic cations from very concentrated solutions. The chelating resin which is the subject of the present invention exhibits properties which are clearly superior to the known aminoalkylphosphonic resins, notably a greatly improved resistance to osmotic shock during use in highly concentrated solutions.
The procedure which is the subject of the present invention makes it possible to produce this resin, the improved properties of which are obtained thanks to its manufacture under strictly controlled, specific conditions, on the basis of a reticulated vinylaromatic matrix with predetermined characteristics.
The present invention further includes the use of the improved resin in the purification of solutions of high osmotic strength, notably the concentrated brines which are used in the electrolytic production of chlorine, chlorates and alkali metal hydroxides. All three of the present industrial procedures used for the manufacture of chlorine by means of the electrolysis of concentrated alkali metal chloride brines require suitable monitoring of the amounts of calcium and magnesium which are present as impurities in these brine solutions. The calcium concentration is generally reduced to about 10 mg/l for mercury cells and to about 3 to 5 mg/l for effective operation of diaphragm cells. The calcium concentration must however be reduced to less than 0.05 mg/l, preferably as low as 0.02 mg/l, for the effective operation of membrane cells. Furthermore, for membrane cells, the magnesium concentration should preferably be reduced to less than 0.005 mg/l. An excess of calcium and magnesium above the indicated limits reduces not only the performance but also the life-span of the membranes. High purity brines are similarly required in the electrolytic production of chlorates.
The amount of calcium in the brines can be reduced to 2 to 10 mg/l by suitable chemical pretreatments involving decanting and filtration. Greater reduction in the calcium level can only be practically and economically realized by the use of ion-exchange chelating resins.
An object of the present invention is to provide a procedure for the substantial elimination of the calcium and magnesium ions present in electrolysis saline brines, up to a high degree of purity (in particular, a calcium content below 0.05 mg/l, preferably about 0.02 mg/l) by means of ion-exchange resins with chelating properties used in a simple, conventional apparatus.
In ion-exchange practice, it is well known that if one passes a solution containing monovalent ions (for example, sodium) and divalent ions (for example, calcium), which is the case in electrolytic brines, over an ion-exchange resin in a suitable ionic form and completely free of divalent ions, one obtains at the outlet of the ion-exchange column, the said solution which no longer contains divalent ions. However, in order for this procedure to be technically and economically viable, it is necessary that the ion-exchanger considered should have a sufficiently high useful capacity, which is to say that the real quantity of divalent ions fixed under practical operating conditions must be sufficiently high, when it is in equilibrium with the inflowing solution. If this useful capacity is low, the volume of solution free from calcium ions for example, obtained per exchange cycle, will be reduced and the procedure will not be of technical or economic value. This useful capacity is strongly influenced by various factors such as the exchange kinetics, the theoretical total capacity of the ion-exchange resin, and the concentration of the ions in solution and by the selectivity of the ion-exchanger in relation to the ions to be eliminated.
A recent procedure is known for the removal of calcium and magnesium ions from alkaline electrolysis brines in which it was proposed to use (a) chelating resins of the type having aminoacetic groups grafted onto styrene-butadiene copolymers, styrene-divinylbenzene copolymers or N-glycine-glycidylmethacrylate copolymers, or onto polymers of epichlorohydrin or preferably (b) chelating compounds such as aminoacetic acid derivatives, adsorbed on inert supports such as activated carbon, silica gel, alumina, zeolite or adsorbent synthetic polymers. These chelating compounds adsorbed on an inert support would exhibit the following advantages: greater mechanical strength and chemical stability, lower production cost, and improved performances. Nevertheless, the use of the cited products does not greatly facilitate the reduction of the content of calcium and magnesium ions in the brine to below 10 mg/l.
The chelating resins of the alkylaminophosphonic type are known. Nevertheless, the use of such resins (such as described in French Patent Application No. 74.25610) is disadvantageous due to the fact that they have insufficient mechanical resistance to the osmotic shocks which occur during conversion of the resin to and from the regenerated and saturated forms. The high percentage of broken and cracked beads which thus occurs severely inhibits the economic use of such resins on an industrial scale for the treatment of brines in the chlorine and chlorate industries.